The measurement of resistivity or sheet resistance in semiconductor doped regions, such as tubs (or wells), which are built on semiconductor substrates is a standard parameter characterized in CMOS technologies. Typically, a tub sheet resistance is measured under field oxide. This parameter is generally the measurement of interest when the area is being used as a resistor in an analog circuit or as an input resistor in an electrostatic discharge (ESD) protection circuit. The sheet resistance of the tub under field oxide is not the only value of interest, however. Tub resistance will typically be different under opposite-polarity diffusion areas or gate regions as compared to measurements under only field oxide. The values of tub resistance under opposite-polarity diffusion areas or gate regions are important in the determination of parasitic properties such as the effectiveness of tub contacts in latch up protection. However, the tub resistance under opposite-polarity diffusion areas or gate regions is typically not measured due to the lack of suitable test structures.
The origin of different tub sheet resistances under these field oxides, diffusion areas and gate structures may be attributed to the major differences in the doping profiles at the top surface of the tub for these conditions. Under field oxide, the tub doping may also be affected by any additional channel-stopping implants that are used under the field oxide. The tub sheet resistance will also be affected by the depletion width of the junction, which depends on the voltage difference between the tub and the substrate.
Under opposite-polarity diffusion areas (e.g., a p+ diffusion area for an N-tub), threshold voltage adjustment implants and the top of the tub doping will be counter doped by the opposite polarity diffusion thereby increasing the resistance of the tub. Depletion effects derived from both the opposite-polarity diffusion area to the tub and the tub to substrate junctions will affect the tub sheet resistance under the opposite-polarity diffusion area. Under a gate region (e.g., a p-channel gate region for an N-tub), channel-stop implants will not typically be included in the tub doping profile. However, threshold voltage adjustment implants will affect the tub doping profile. The tub sheet resistance will also be affected by the substrate voltage and by the gate voltage as the surface under the gate oxide is moved from accumulation, to depletion and into inversion.
The van der Pauw measurement technique has been used to accurately measure the sheet resistance of an arbitrarily shaped conductor provided that several measurement criteria are met. The measurement contacts to the sample to be measured must be on the outer edges of the sample, and the contact size must be small compared to the sample area. The sample must be of generally uniform thickness and must be singly connected containing no isolated holes.
Two voltage-current measurements are then made using the same four contact points on the periphery of the sample. First, a current is forced between two adjacent contacts, while the resulting voltage is measured between the two remaining contacts. The measurement is then repeated after shifting all current and voltage contacts clockwise or counter clockwise by one contact. The resulting two calculated resistances are then averaged to provided an average measured resistance value. The sheet resistance of the sample may then be calculated to be 4.532 times the average measured resistance value. This technique has been successfully applied to the measurement of tub sheet resistance for tubs formed under field oxide.
In summary, the resistivity or sheet resistance of a tub or well may be modified considerably due to many different sources. Some of these include channel-stopping implants, the depletion width of a junction, threshold voltage adjustment implants and applied substrate and gate voltages. Unfortunately, the current van der Pauw test devices can only measure the tub sheet resistance under field oxide.
Accordingly, what is needed in the art is a way to accurately determine the resistivity of a doped area of a semiconductor wafer under a diffusion of opposite type or under a MOS transistor gate.